1. Field of the Invention
This invention relates generally to telecommunications switching apparatus, and more particularly to a high voltage, high current, low operating power ringing access relay for telecommunications switching applications in remote environments.
2. Description of Related Art
A solid state switch connected between a ringing signal generator and a ring device in telecommunications systems has a known minimum breakdown voltage requirement determined by the maximum clamp voltage of the external protection device connected from the ring device to ground plus the maximum ring voltage having an opposite polarity. In certain applications, the minimum breakdown voltage of the ringing access switch is approximately 500 volts. However, when utilizing components having a 320 volt breakdown capability, the high voltage junctions must necessarily be placed in series to meet this requirement.
In addition to the breakdown requirement in the OFF state, the ringing access switch must survive fault conditions in the ON state without degradation of performance. Surge currents through this switch resulting from, for example a lightening strike, can reach over 2 amps depending upon the amplitude of the lightening pulse and the impedance in the ringing circuit path.
To minimize current flow through a ringing access switch, the switch design can include current limiting circuitry to permit a maximum level of conduction that does not interfere with peak ringing signal currents, approximately 150 mA. However, because of the breakdown voltage requirement, current limiting of two series bidirectional switches is required. This presents a very complicated and costly design problem, for each series switch must share the same voltage and power.
Previous designs consisting of ringing access switches implemented in integrated circuits have exhibited inherent limitation of inadvertent turn-on in the presence of fast voltage transients at the switch terminals. This dV/dt sensitivity can cause the switch to become latched in the wrong state, requiring a reset of the device. Designs that improve dV/dt sensitivity to eliminate the inadvertent turn-on of the switch requires relatively higher bias current and operating power. However, these designs cannot be used in applications where low operating power is of primary importance.